In a transmission electron microscope, electrons transmitted through a specimen suffer from energy loss that is peculiar to one or more elements composing the specimen. To overcome this shortcoming, the electrons transmitted through the specimen are passed through an energy filter for analyzing energy of those electrons, separating only the electrons suffering from the specific energy loss, and forming an image of the separated electrons. The formed image corresponds to a mapping image of one or more specific elements contained in the specimen. Further, the use of only the electrons having specific energy for forming an image allows the energy loss of the electrons caused by the thickness of the specimen to be restricted to only specific electrons. The resulting image has excellent contrast.
For the electron energy filter used for this kind of purpose, there have been known an omega type energy filter (U.S. Pat. No. 4,740,704) and an alpha type energy filter (U.S. Pat. No. 4,760,261).
The omega type energy filter is composed of three electromagnets as shown in FIG. 2. The first electromagnet 1 has an opposite deflecting direction to the second and the third electromagnets 2 and 3. Incident electrons 4 are traced like an omega (.OMEGA.) and are fired in the same direction as the incident one, for selecting only the electrons having specific energy loss. As shown, a numeral 5 denotes a crossover point. A numeral 6 denotes an inlet image surface. A numeral 7 denotes an outlet image surface. A numeral 8 denotes an energy dispersion surface.
The alpha type energy filter is composed of three electromagnets 11, 12 and 13 having the same deflecting directions as each other as shown in FIG. 3. Incident electrons 4 are traced like an alpha (.alpha.) and finally outgoes in the same direction as the incident one, for selecting only the electrons that suffer from specific energy loss. Another kind of alpha type energy filter is shown in FIG. 4 (Perez, J. P., Sirven, J., Sequela, A., and Lacaze, J. C., Journal de Physique, (Paris), 45, Coll. Cs, 171 to 174 (1984)). This energy filter is constructed so that an electromagnet 14 having a deflecting angle of 70.degree. is located as opposed to the other electromagnet 15 having a deflecting angle of 220.degree. with narrow middle space 16, in which those electromagnets yield the corresponding magnetic field intensities. In the construction, an incident electron beam 4 goes around the inside of the filter for analyzing the energy of the electron beam.